A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To ensure that the pattern of a patterning device is transferred onto the correct target position on a substrate, it is important that the substrate is accurately aligned with respect to the lithographic apparatus. Especially in the manufacturing of ICs, which usually contain many individual layers (e.g., 30 layers), it is extremely important that the several individual layers are lined up with respect to each other very accurately. When the line up of the individual layers of an IC is not very accurate the IC might not be an operable IC. Overlay is defined as being the accuracy within which a pattern is imaged on a substrate in relation to the patterns that have previously been imaged on the substrate. As in the manufacture of ICs ever smaller features are imaged, the overlay requirements and hence the necessary accuracy of the alignment process, become stricter.
Lithographic apparatus are known to use multiple alignment systems to align the substrate with respect to the lithographic apparatus. Reference is made in particular to the European patent application EP 1 372 040 A1, which document is hereby incorporated by reference. EP 1 372 040 A1 describes an alignment system using a self-referencing interferometer that produces two overlapping images of an alignment marker. These two images are rotated over 180° with respect to each other. EP 1 372 040 A1 further describes the detection of the intensity variation of the interfering Fourier transforms of these two images in a pupil plane. These intensity variations correspond to a phase difference between different diffraction orders of the two images, and from this phase difference positional information is derived, which is required for the alignment process.
A disadvantage of the known alignment system is that the self-referencing interferometer in the alignment measurement system may be relatively expensive as its optical design requires the use of specially manufactured optical components. A further disadvantage of the known alignment system is that the known self-referencing interferometer is usually quite bulky. This bulkiness may cause unwanted side-effects in the alignment process, such as low bandwidth vibrations modes that can impede the accuracy of the alignment process.